1. Field of the Invention
The present invention relates to a mixture positive-electrode active material having an enlarged available state of charge (SOC) area and improved output characteristics at a low voltage level, a lithium secondary battery including the same, and a method for manufacturing the same.
2. Related Art
As the development of techniques and demand for mobile devices are increasing, the demand of secondary batteries as an energy source has been rapidly growing. Among secondary batteries, a lithium secondary battery having a high energy density and voltage, a long cycle life span, and a low self-discharge rate, has been commercialized and widely used. Also, as people are increasingly interest in the environment issues, research for an electric vehicle, a hybrid electric vehicle, or the like, which may replace the vehicles, such as a gasoline vehicle, a diesel vehicle, or the like, using fossil fuel, one of major causes of air pollution, has been greatly conducted. Recently, research into the use of a lithium secondary battery having a high energy density and discharge voltage as a power source of an electric vehicle, a hybrid electric vehicle, or the like, is actively ongoing and a lithium secondary battery is partially in a commercialization stage.
In particular, researches for developing a positive electrode material of a large capacity lithium secondary battery to be used for an electric vehicle are comprehensively carried out to replace currently used LICoO2.
In case of LiCoO2, the existing typical positive material, has reached its limit in an increase in an energy density and a practical use of output characteristics, and in particular, when LiCoO2 is used in a high energy density application field, its structure is denatured at a high charge state due to a structural instability and oxygen in the structure is discharged to cause an exothermic reaction with electrolyte in a battery to mainly cause a battery explosion. Thus, in order to improve the instability of LiCoO2, the use of a lithium-containing manganese oxide such as LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure, or the like, has been considered, and recently, a great deal of researches into the use of three-component system layered oxides of LiNixMnyCo1-x-yO2 have been done.
Li[Ni1/3Co1/3Mn1/3]O2, the most typical layered oxide among the three-component layered oxides, is changed from Ni2+ to Ni3+ or Ni4+ according to the depth of charge when charging is performed. However, unlike stable Ni2+, Ni3+ or Ni4+ (in particular Ni4+) loses lattice oxygen so as to be reduced to Ni2+ due to instability, and the lattice oxygen reacts with electrolyte to change the surface qualities of an electrode or increase a charge transfer impedance of the surface of the electrode to reduce the capacity or degrade high rate capability.
In order to improve the problem of the three-component layered oxide, research for mixing a metal oxide having an olivine structure to the 3-component positive electrode active material has been conducted, but the metal oxide having the olivine structure has a low reversible capacity and low output characteristics due to low electric conductivity, having a problem in that a volume energy density is low in spite of the advantages of a low cost and high security. In particular, when the three-component layered oxide and the metal oxide having the olivine structure are mixed, a rapid voltage drop is caused in the vicinity of 3.6 V to 3.4 V during discharging due to the difference in the operation voltage, so the output is sharply dropped in a state of charge (SOC) area of the corresponding portion.
Japanese Laid Open Publication No. 2001-307730 discloses a non-aqueous electrolyte secondary battery using a mixture, which is obtained by a second lithium compound such as a lithium-containing cobalt oxide, a lithium-containing nickel cobalt oxide, or the like, to a first lithium compound including a lithium-containing olivine type phosphate, as a positive electrode active material. However, the lithium secondary battery according to this document still has the problem of degradation of the instantaneous output because there is a portion in which voltage is rapidly dropped at an end portion of an operation voltage of the second lithium compound due to the difference between the operation voltages of the two mixed materials.
These problems may possibly become serious when the lithium secondary battery is used as a power source of midsize and large devices such as an electric vehicle in which maintaining of the output in the available SOC area is essential, so research into a secondary battery which does not have a rapid output drop in the available SOC area while maintaining a high level of security and exhibits a high output even at a low SOC area is urgently required.